1. Field of the Invention
The present invention relates to a continuously variable transmission for a vehicle. More particularly, the present invention relates to a continuously variable transmission for a vehicle which improves fuel economy and power performance as a consequence of realizing two forward speeds and one reverse speed basically by using a plurality of externally-meshed gear sets and a variable shift apparatus, changing speed ratios continuously within basic shift-speeds, and enlarging a range of speed ratios.
2. Description of Related Art
Generally, a continuously variable transmission is configured to receive torque output from an engine through power delivery means of fluid type such as a torque converter or a fluid coupling, to change the torque according to a speed ratio which changes continuously and is achieved by a variable shift apparatus, and to transmit the changed torque to an output shaft.
The variable shift apparatus includes first and second variable pulleys disposed respectively on a driving shaft and a driven shaft, and a metal belt connecting the first and second variable pulleys. The variable shift apparatus is adapted to change the speed ratios continuously by controlling widths of the first and second variable pulleys connected through the metal belt by hydraulic pressure.
If a target shift-speed according to a driving condition such as a vehicle speed, an engine rotation speed, and a load is calculated by using a predetermined shift pattern, an opening of a shift-speed control valve connected to the first and second variable pulleys is controlled so as to achieve the target shift-speed.
Recently, a continuously variable transmission using an auxiliary shift apparatus has been vividly researched. The continuously variable transmission using the auxiliary shift apparatus includes shift-speed control means provided with at least one planetary gear set, the variable shift apparatus, and at least one externally-meshed gear set. The continuously variable transmission using the auxiliary shift apparatus realizes two forward speeds and one reverse speed basically and changes speed ratios continuously within basic shift-speeds.
However, since the continuously variable transmission using the auxiliary shift apparatus includes at least one planetary gear set, structure thereof may be complex and mountability may be deteriorated due to increase of volume and weight. In addition, fuel economy may be deteriorated due to increase of the weight.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.